This invention relates generally to detecting a boil dry condition in a utensil being heated on a cooking appliance and more particularly to boil dry detection in glass-ceramic cooking appliances.
The use of glass-ceramic plates as the cooking surface in cooking appliances such as cooktops and ranges is well known. Such cooking appliances (referred to herein as glass-ceramic cooktop appliances) typically include a number of heating units mounted under the glass-ceramic plate, one or more sensors for measuring the glass-ceramic temperature, and an electronic or electro-mechanical controller. The glass-ceramic plate presents a pleasing appearance and is easily cleaned in that its smooth, continuous surface lacks seams or recesses in which debris can accumulate. The glass-ceramic plate also prevents spillovers from falling onto the heating units below. The controller controls the power applied to the heating units in response to user input and input from the temperature sensors.
In one known type of glass-ceramic cooktop appliance, the glass-ceramic plate is heated predominantly by radiation from one or more of the heating units disposed beneath the plate. The glass-ceramic plate is sufficiently heated by the heating unit to heat utensils placed on it primarily by conduction from the heated glass-ceramic plate to the utensil. Another type of glass-ceramic cooktop appliance uses a heating unit that radiates substantially in the infrared region in combination with a glass-ceramic plate that is substantially transparent to such radiation. In these appliances, a utensil placed on the cooking surface is heated partially by radiation transmitted directly from the heating unit to the utensil, in addition to conduction from the glass-ceramic plate. Such radiant glass-ceramic cooktop appliances are more thermally efficient than other glass-ceramic cooktop appliances and have the further advantage of responding more quickly to changes in the power level applied to the heating unit. Yet another type of glass-ceramic cooktop appliance inductively heats utensils placed on the cooking surface. In this case, the heating unit is a coil connected to an RF generator; the coil emits RF energy when activated. The utensil, which comprises an appropriate material, absorbs the RF energy and is thus heated.
In each type of glass-ceramic cooktop appliances, provision must be made to avoid overheating the glass-ceramic plate. For most glass-ceramic materials, the operating temperature should not exceed approximately 600-700xc2x0C. for any prolonged period. Under normal operating conditions, the temperature of the glass-ceramic plate will generally remain below this limit. However, conditions can occur which can cause this temperature limit to be exceeded. Commonly occurring examples include operating the appliance with a small load or no load (i.e., no utensil) on the cooking surface, using badly warped utensils that make uneven contact with the cooking surface, and operating the appliance with a shiny and/or empty utensil.
To protect the glass-ceramic plate from extreme temperatures, a control system is utilized in which temperature sensors provide a signal indicative of the glass-ceramic temperature to the appliance""s controller. If the glass-ceramic plate approaches its maximum temperature, a special control mode, known as the thermal limiter mode, is activated. In the thermal limiter mode, the controller reduces power to the heating units to maintain the temperature of the glass-ceramic cooking surface at a relatively constant, safe temperature.
Another concern with cooking appliance generally is a boil dry condition. A boil dry condition occurs when all the liquid contents of a heated utensil evaporate during the boil phase. This commonly happens when a utensil is inadvertently left on a hot cooking surface or otherwise overheated. A boil dry condition can cause burned food, utensil damage and potential fire hazards. Accordingly, automatic detection of a boil dry condition is a desirable feature in cooking appliances.
In glass-ceramic cooktop appliances, it is known to use the glass ceramic temperature to determine when a utensil has boiled dry. Specifically, when a utensil containing water or another liquid is placed on a glass-ceramic cooking surface and the burner is turned on, the glass-ceramic temperature initially increases rapidly. The glass-ceramic temperature will continue to rise until the utensil contents come to a boil. During the boil phase, the utensil contents will boil off at a steady temperature and remove excess heat via evaporation. With this steady heat removal, the glass-ceramic temperature also reaches a steady state value some time after the contents come to a boil. However, when the liquid completely boils off, there is a sudden drop in heat removal from the pan, and consequently, the glass-ceramic temperature increases rapidly. This temperature rise is thus indicative of the boil dry condition.
This method of boil dry detection generally works well while the cooking appliance is in its standard operating mode. But under the thermal limiter mode, the glass-ceramic plate is being maintained at a relatively constant temperature by the controller. Therefore, the glass-ceramic temperature will not rise when upon a boil dry condition. Accordingly, it would be desirable to be able to automatically detect boil dry conditions while in the thermal limiter mode.
The above-mentioned need is met by the present invention, which provides a boil dry detection system for a glass-ceramic cooking appliance having at least one heating unit disposed under a glass-ceramic plate and a power source for providing power to the heating unit. The boil dry detection system includes means for providing a signal representative of the level of power being supplied to the heating unit. A controller for controlling the power source so as to prevent the glass-ceramic plate from exceeding a maximum temperature is arranged to receive the power level signal. The controller provides a boil dry indication in response to a decrease in the power level signal.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.